Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products such as mobile phones, laptop computers, MP3 players and tablets. Requirements of the mobile phone industry, for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers.
The result of this is the emergence of micro-electrical-mechanical-systems (MEMS) based transducer devices. These may be, for example, capacitive transducers for detecting and/or generating pressure/sound waves or transducers for detecting acceleration. MEMS capacitive microphones typically comprise a first electrode, which is moveable with respect to a second fixed electrode in response to incident acoustic waves. The first electrode may, for example, be supported by a flexible membrane. By measuring changes in the capacitance between the electrodes, the incident acoustic signals can be detected. In use the electrodes of the MEMS microphone may be biased by biasing circuitry and the measurement signal may be amplified by amplifier circuitry such as a low-noise amplifier. MEMS transducers may also be designed to operate in the reverse mode of operation, in which electrical signals are applied to one or both of the electrodes to drive motion of the flexible membrane and so generate pressure/sound waves.
Although the process for manufacturing MEMS components has improved with the considerable research and development that has taken place in recent years, the uniformity of devices produced by such processes is still a significant issue for the industry. Inevitably, minor differences will exist between MEMS components even if they are manufactured by the same process. In the field of MEMS capacitive transducers, this can result in variation between the capacitance of individual components.
Further, it is known that the quiescent capacitance of a MEMS transducer (i.e. the capacitance when the transducer is not subject to incoming pressure waves, or driving input signals) may change over time, based on a number of factors such as the amount of use the transducer is subject to, the amplitude of signals used to drive the transducer, or the amplitude of pressure/sound waves detected by the transducer, and environmental conditions such as temperature and humidity.
As such, it is useful to be able to determine the capacitance value of the MEMS transducer, following manufacture (e.g. using external test circuitry in the laboratory or manufacturing plant), during use (e.g. using test circuitry on-chip, within the same package in which the transducer is housed or, more generally, within the same host device in which the MEMS transducer is used) or both. The capacitance value may be used to adjust the biasing voltage applied to the electrodes, or otherwise calibrate the input/output signals applied to or generated by the MEMS transducer so as to achieve a consistent performance from sample to sample or over time.
Methods and apparatus for determining the capacitance of a MEMS transducer are therefore required.